1. Field of the Invention
The present invention relates to an ultrathin copper foil with a carrier and a printed circuit board using the ultrathin copper foil with the carrier, more particularly relates to an ultrathin copper foil with a carrier suitable for a printed circuit board for high density ultrafine circuit (fine pattern) applications, a multi-layer printed circuit board, and a chip-on-film use circuit board.
2. Description of the Related Art
Usually, in the copper foil used for a printed circuit board forming the base of a printed circuit board, a multi-layer printed circuit board, a chip-on-film use circuit board, or the like, a surface on the side heat pressed to a plastic substrate or the like is formed as a roughened surface and this roughened surface is used to manifest an anchor effect with respect to the substrate so as to raise a bond strength between the substrate and the copper foil and thereby to secure reliability as a printed circuit board.
Further, recently, copper foil with a resin layer formed by covering the roughened surface of copper foil by a bonding use resin such as an epoxy resin in advance and using the bonding use resin as an insulating resin layer of a semi-cured state (B stage) is used as copper foil for forming interconnects. The side with the insulating resin layer is hot pressed to a substrate to form a printed circuit board. Such printed circuit boards are stacked in multiple layers to thereby produce a builtup circuit board. A builtup circuit board is one type of multi-layer printed circuit board and is obtained by forming an insulating layer and a conductive pattern on the insulating substrate one layer at a time in that order, plating holes (vias) formed by a laser process or a photo process, and stacking such circuit layers while connecting the layers.
In this circuit board, the vias can be made finer corresponding to the higher integration of various types of electronic devices. Therefore, interconnect patterns are increasingly being required to be made finer in line widths and line pitches. For example, in the case of printed circuit boards used in semiconductor packages, provision of printed circuit boards having high density ultrafine interconnects having line widths and line pitches of approximately 30 μm is being demanded.
When using thick copper foil as the copper foil for such a fine pattern printed circuit board, an etching time at the time of formation of the interconnects and circuits by etching becomes long. As a result, a verticalness of side walls of the formed interconnect patterns is lost. When the interconnect line widths of the formed interconnect patterns are narrow, this sometimes leads to disconnection. Accordingly, as the copper foil used for fine pattern applications, copper foil having a thickness of 9 μm or less is required. At present, copper foil having a thickness of about 5 μm is most frequently used. Further thinner copper foil is being sought.
However, such thin copper foil (hereinafter sometimes also referred to as “ultrathin copper foil”) has a weak mechanical strength, is easily wrinkled or creased at the time of the production of the printed circuit board, and sometimes even breaks. Therefore, as the ultrathin copper foil used for the fine pattern applications, use is made of an ultrathin copper foil with a carrier formed by directly electrodepositing an ultrathin copper foil layer on one surface of metal foil serving as a carrier (hereinafter, referred to as “carrier foil”) via a release layer.
As explained above, the copper foil having a thickness of 5 μm which is frequently used at present is provided as the ultrathin copper foil with the carrier.
The ultrathin copper foil with the carrier has a carrier foil on one surface of which a release layer and ultrathin copper foil deposited by electric copper plating are formed in that order. The outermost layer surface of the ultrathin copper foil made of the electric copper plating is finished to a roughened surface.
As the release layer formed on the one surface of the carrier foil, an organic coating film, Cr metal, Cr alloy, chromate, etc. are usually used, but in recent years in interconnect substrates using a polyamide or other high temperature plastic etc. as the insulating substrate, the conditions such as the pressing temperature of the copper foil and the substrate or the curing temperature are high temperature, therefore an organic release layer can no longer be peeled off. Therefore, an organic coating film cannot be used, and a metal-based release layer is used instead.
As the metal for forming the release layer, as described before, a Cr metal, Cr alloy, and chromate form the mainstream. However, when using Cr for the release layer, blistering occurs in the high temperature process of production of the interconnect substrates, variation occurs in the release properties, and some problem arises in stability of production of the interconnect substrates.
Further, some metals such as Cr reportedly exert an adverse influence upon the human body. A future ban on use of these metals may even be expected. Accordingly, in actual circumstances, the use of a metal such as Cr must be prevented as much as possible.
Summarizing the problems in the art, as explained above, a release layer using Cr results in insufficient stability of high temperature production of the interconnect substrates. The appearance of the ultrathin copper foil with the carrier using a release layer not using Cr metal liable to have an effect on the human body or suppressing the content of Cr to the smallest limit and enabling easy release even under a high temperature has therefore been desired.